1. Field of the Invention
The present invention relates to communication systems using narrow or very narrow band channels, so that the number of channels available for different types of communication systems (for example, cellular telephones, pagers, police/fire/rescue) can be increased. Specifically, the invention relates to a system and method for transmitting and receiving signals using frequency modulation (FM) or phase modulation (PM) on narrowband or very narrowband channels.
2. Related Art
Existing frequency or phase modulation systems have advantages over amplitude modulation systems. These advantages include excellent sound quality, higher signal-to-noise ratio, and the availability of communication channels in the electromagnetic spectrum, which overcomes the disadvantages of wide-band frequency modulation. As a result, manufacturers of radio communication systems and communication research centers have not encouraged improvements to existing FM/PM systems.
In recent years, with the expansion of pagers, cellular telephones, the Internet and other computer networks, the VHF and UHF bands have become very congested and the demand for more channels has forced manufacturers to use higher frequency bands. The higher baud rate, digital data transmission requires a wide band. To reduce the bandwidth of the digital communication channels, data compression techniques have been implemented. These techniques do not work, however, for on-line audio and video communication because of the time delay caused by the compression and decompression processes.
In order to satisfy the high demand for more communication channels, technology must be developed that will dramatically reduce the bandwidth of the communication channel, transmit more bits per second per hertz of bandwidth, and increase the power of the transmitted information without adversely affecting sound quality.